Electrolytes having improved stability

ABSTRACT

The invention provides an electrochemical cell having an electrolyte which comprises a solute, a solvent, and an additive. The additive is a dialkylamide. The dialkylamide lessens the extent of decomposition of the solute, which is a lithium salt. The ionic species of the lithium salt are thereby preserved. The additive also prevents damage to active material by absorbing excess charge energy below the breakdown potential of the active material.

FIELD OF THE INVENTION

[0001] This invention relates to electrolytes which function as a sourceof alkali metal ions for providing ionic mobility and conductivity. Theinvention more particularly relates to electrolytic cells where suchelectrolytes function as an ionically conductive path betweenelectrodes.

BACKGROUND OF THE INVENTION

[0002] Electrolytes are an essential member of an electrolytic cell orbattery. In one arrangement, a battery or cell comprises an intermediateseparator element containing an electrolyte solution through whichlithium ions from a source electrode material move between cellelectrodes during the charge/discharge cycles of the cell. The inventionis particularly useful for making such cells in which the ion sourceelectrode is a lithium compound or other material capable ofintercalating lithium ions, and where an electrode separator membranecomprises a polymeric matrix made ionically conductive by theincorporation of an organic solution of a dissociable lithium salt whichprovides ionic mobility.

[0003] Early Lithium Metal Cells

[0004] Early rechargeable lithium cells utilized lithium metalelectrodes as the ion source in conjunction with positive electrodescomprising compounds capable of intercalating the lithium ions withintheir structure during discharge of the cell. Such cells relied, for themost part, on separator structures or membranes which physicallycontained a measure of fluid electrolyte, usually in the form of asolution of a lithium compound, and which also provided a means forpreventing destructive contact between the electrodes of the cell.Sheets or membranes ranging from glass fiber, filter paper or cloth tomicroporous polyolefin film or nonwoven organic or inorganic fabric havebeen saturated with solutions of an inorganic lithium compound, such asLiClO₄, LiPF₆, or LiBF₄, in an organic solvent to form suchelectrolyte/separator elements. The fluid electrolyte bridge thusestablished between the electrodes has effectively provided thenecessary Li+ ion mobility at conductivities in the range of about 10⁻³S/cm.

[0005] Ion, Rocking Chair Cells and Polymer Cells

[0006] Lithium metal anodes cause dendrite formation during chargingcycles which eventually leads to internal cell short-circuiting. Somesuccess has been achieved in combatting this problem through the use oflithium-ion cells in which both electrodes comprise intercalationmaterials, such as lithiated metal oxide and carbon (U.S. Pat. No.5,196,279), thereby eliminating the lithium metal which promotes thedeleterious dendrite growth. Another approach to controlling thedendrite problem has been the use of continuous films or bodies ofpolymeric materials which provide little or no continuous free path oflow viscosity fluid in which the lithium dendrite may propagate. Thesematerials may comprise polymers, e.g., poly(alkene oxide), which areenhanced in ionic conductivity by the incorporation of a salt, typicallya lithium salt such as LiClO₄, LiPF₆, or the like. A range of practicalionic conductivity, i.e., over about 10⁻⁵ to 10⁻³ S/cm, was onlyattainable with these polymer compositions at well above roomtemperature, however. (U.S. Pat. Nos. 5,009,970 and 5,041,346.)

[0007] “Solid” and “Liquid” Batteries of the Prior Art

[0008] More specifically, electrolytic cells containing an anode, acathode, and a solid, solvent-containing electrolyte incorporating aninorganic ion salt were referred to as “solid batteries”. (U.S. Pat. No.5,411,820). These cells offer a number of advantages over electrolyticcells containing a liquid electrolyte (i.e., “liquid batteries”)including improved safety factors. Despite their advantages, themanufacture of these solid batteries requires careful process control tominimize the formation of impurities. Solid batteries employ a solidelectrolyte matrix interposed between a cathode and an anode. Theinorganic matrix may be non-polymeric [e.g., β-alumina, silver oxide,lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene)polymers] whereas the organic matrix is typically polymeric. Suitableorganic polymeric matrices are well known in the art and are typicallyorganic polymers obtained by polymerization of a suitable organicmonomer as described, for example, in U.S. Pat. No. 4,908,283.

[0009] Examples of solvents known in the art are propylene carbonate,ethylene carbonate, γ-butyrolactone, tetrahydrofuran, glyme(dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane,sulfolane, diethoxyethane, and the like. These are examples of aprotic,polar solvents.

[0010] Heretofore, the solid, solvent-containing electrolyte hastypically been formed by one of two methods. In one method, the solidmatrix is first formed and then a requisite amount of this material isdissolved in a volatile solvent. Requisite amounts of the inorganic ionsalt and the electrolyte solvent (usually a glyme and the organiccarbonate) are then added to the solution. This solution is then placedon the surface of a suitable substrate (e.g., the surface of a cathode)and the volatile solvent is removed to provide for the solidelectrolyte. In another method, a monomer or partial polymer of thepolymeric matrix to be formed is combined with appropriate amounts ofthe inorganic ion salt and the solvent. This mixture is then placed onthe surface of a suitable substrate (e.g., the surface of the cathode)and the monomer is polymerized or cured (or the partial polymer is thenfurther polymerized or cured) by conventional techniques (heat,ultraviolet radiation, electron beams, etc.) so as to form the solid,solvent-containing electrolyte. When the solid electrolyte is formed ona cathodic surface, an anodic material can then be laminated onto thesolid electrolyte to form a solid battery (i.e. an electrolytic cell).

[0011] More recently, a highly favored electrolyte/separator film isprepared from a copolymer of vinylidene fluoride andhexafluoropropylene. Methods for making such films for cell electrodesand electrolyte/separator layers are described in U.S. Pat. Nos.5,418,091; 5,460,904; and 5,456,000 assigned to Bell CommunicationsResearch, each of which is incorporated herein by reference in itsentirety. A flexible polymeric film useful as an interelectrodeseparator or electrolyte member in electrolytic devices, such asrechargeable batteries, comprises a copolymer of vinylidene fluoridewith 2 to 25% hexafluoropropylene. The film may be cast or formed as aself-supporting layer retaining about 20% to 70% of a high-boilingsolvent or solvent mixture comprising such solvents as ethylenecarbonate or propylene carbonate. The film may be used in such form orafter leaching of the retained solvent with a film-inert low-boilingsolvent to provide a separator member into which a solution ofelectrolytic salt is subsequently imbibed to displace retained solventor replace solvent previously leached from the polymeric matrix.

[0012] Electrolyte Breakdown

[0013] Regardless of which technique is used in preparing anelectrolyte/separator, a recurring problem has been the loss ofeffectiveness of the electrolyte. The electrolyte has been observed tochange color, evidencing a degradation that has not been wellunderstood. There is presently no effective means to maintain the usefulserviceability of the electrolyte.

[0014] In view of the above, it can be seen that it is desirable to havea novel, economical means for maintaining electrolyte integrity; andwhich maintains cell capacity in a variety of electrolyte/separatorconfigurations, including those described above as exemplary.

SUMMARY OF THE INVENTION

[0015] The present invention provides an additive for an electrolytesolution of an electrochemical cell. The additive provides anelectrolyte solution stabilized against decomposition during storage andduring cyclic operation of an electrochemical cell. The additive is adialkylamide, desirably a N,N-dialkylamide, and preferably isN,N-dimethylacetamide (DMAC). Advantageously, the additive preventsundesired decomposition of cell components, and particularly electrolytesolution components. Such undesired decomposition is evidenced by achange in color of the electrolyte solution, and may also result inundesired gaseous by-products. Gaseous by-products lead to volumetricexpansion and possible rupture of the cell. The additive is usable witha variety of carbonaceous and metal oxide electrode active materials,providing improved performance without decomposition, which wouldotherwise occur, absent the additive.

[0016] In addition, the DMAC additive breaks down at potentials at ornear an overcharge condition, nominally at or over about 4.4 volts.Thus, in a condition at or near cell overcharge condition, the DMACadditive aborbs excess charge energy by degrading at or just about 4.4volts. This protects the active material from electrochemical damage bypreventing the attainment of the damage threshold voltage, about 4.7volts or higher, for lithium metal oxide active materials such aslithium manganese oxide, lithium cobalt oxide and lithium nickel oxide.

[0017] In one embodiment, the invention provides an electrochemical cellhaving an electrolyte which comprises a solute, a solvent, and theadditive of the invention. The solute is a salt of lithium. The solventcomprises one or more aprotic, polar solvents. The dialkylamide of theinvention is usable with a variety of solvents and salts. Exemplarysolvents are carbonates; lactones; propionates; five member heterocyclicring compounds; and organic solvent compounds having a low alkyl (1-4carbon) group connected through an oxygen to a carbon, and comprisingC—O—C bonds. Exemplary solvents are selected from the group consistingof propylene carbonate (PC), ethylene carbonate (EC), methyl ethylcarbonate (MEC), also referred to as ethyl methyl carbonate (EMC)diethyl carbonate (DEC), dipropyl carbonate (DPC), dimethyl carbonate(DMC), butylene carbonate (BC), dibutyl carbonate (DBC), and vinylenecarbonate (VC). Among the preferred solvents are EC/DMC, EC/DEC, EC/DPC,and EC/EMC. With these combinations, there may also be used ethylpropionate (EP). Particularly preferred is EC/DMC/DMAC andEC/DMC/DMAC/EP. Any amount of DMAC added to the electrolyte solvent ishelpful. Practical amounts are in the range of up to 20% by weight ofthe solvent mixture. The DMAC additive, from a practical point of view,may be present in the solvent mixture in an amount of 0.1% to 20% byweight of the solvent mixture. A range of 1% to 5% is preferred.

[0018] The DMAC enhances the thermal stability of lithium salts, such asLiPF₆, and also enhances stability of co-solvents. Lithium salts areknown to be subject to decomposition, and DMAC is useful to prevent suchdecomposition. As a result, the ionic species of the salt are preservedfor ion transport. The additive of the present invention inhibits,prevents, or at least reduces and minimizes undesired side reactionwhich causes decomposition of cell components, and also prevents,inhibits, or reduces evolution of gaseous by-products which occur as aresult of such decomposition. Advantageously, the additive of thepresent invention exhibits good performance and is compatible with awide range of salts, solvents, and electrode active materials. Goodperformance is achieved even with carbonaceous electrode active materialand with transition metal electrode active material which show poorperformance when used in comparative conventional cells without theadditive.

[0019] Objects, features, and advantages of the invention include animproved electrochemical cell or battery having improved charging anddischarging characteristics; a large discharge capacity; and whichmaintains its integrity over a prolonged life cycle, as compared topresently used batteries and cells. Another object is to provide anelectrolyte mixture which is stable with respect to electrode activematerials, and which demonstrates high performance, and which does notreadily decompose, evaporate, or solidify. It is also an objective ofthe present invention to provide cells with electrolyte solutionscompatible with other cell components, avoiding problems with undesiredreactivity, decomposition, and gas formation.

[0020] These and other objects, features, and advantages will becomeapparent from the following description of the preferred embodiments,claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a diagrammatic representation of a typical laminatedlithium-ion battery cell structure which is prepared with theelectrolyte salt of the present invention.

[0022]FIG. 2 is a diagrammatic representation of a multicell batterycell structure which is prepared with the electrolyte salt of thepresent invention.

[0023]FIG. 3 is a voltage/capacity plot, showing cumulative capacity(mAh), for a BG-35 graphite carbon electrode cycled with a lithium metalcounter-electrode using constant current cycling at ±0.2 milliamps persquare centimeter, between 0.01 and 2.0 volts, using 19 milligrams ofthe BG-35 active material. The electrolyte was 1 molar LiPF₆ in asolution of ethylene carbonate (EC) and dimethyl carbonate (DMC), 2:1wt. EC:DMC; and including 10% by weight dimethylacetamide (DMAC).

[0024]FIG. 4 is a voltage/capacity plot showing cumulative capacity(mAh) for lithium manganese oxide (LMO) electrode cycled with a lithiummetal counter-electrode using constant current cycling at ±0.2 milliampsper square centimeter, between 3.0 and 4.3 volts, using 30 milligrams ofthe LMO active material. The electrolyte was 1 molar LiPF₆ in a solutionof 2:1 by weight of EC:DMC; and including 10% by weightdimethylacetamide (DMAC).

[0025]FIG. 5 is a two-part graph showing the results of testing a cell,rocking chair battery, having an anode comprising BG-35 active materialcycled with a counter-electrode comprising lithium manganese oxideactive material. FIG. 5A is Coulombic Efficiency and 5B is DischargeCapacity, each Versus Cycles. The cell charge and discharge are at ±1milliamp hour per centimeter square, between 3 and 4.2 volts for 1 to107 cycles. The negative electrode contained 570 milligrams of the BG-35active material and the positive electrode contained 1710 milligrams ofthe lithium manganese oxide active material. The surface area of thepositive electrode was 48 square centimeters and the surface area of thenegative electrode was 48 square centimeters. The electrolyte comprised1% DMAC in 1 molar LiPF₆ EC/DMC. The weight ratio of EC/DMC was 2:1. Theoverall weight ratio of EC/DMC/DMAC was 66:33:1.

[0026]FIG. 6 is a two-part graph showing the results of testing acomparative cell, without any DMAC additive. (Dashed data line). Thecell had an anode comprising BG-35 active material cycled with acounter-electrode comprising lithium manganese oxide active material.The cell charge and discharge are at ±1 milliamp hour per centimetersquare, between 3 and 4.2 volts for 1 to 100 cycles. The negativeelectrode contained 508 milligrams of the BG-35 active material and thepositive electrode contained 1524 milligrams of the lithium manganeseoxide active material. The surface area of the positive electrode was 48square centimeters and the surface area of the negative electrode was 48square centimeters. The electrolyte was 1 molar LiPF₆ in EC/DMC. Theweight ratio of EC/DMC is 2:1. In FIGS. 6A and 6B, the data for thecomparative cell are shown by dashed (--) lines. The data for the cellof Example III is repeated as a solid line for direct comparison. FIG.6A is Coulombic Efficiency and 6B is Discharge Capacity, each VersusCycles.

[0027]FIG. 7 is voltage/capacity plot taken under the same conditions asFIG. 4, except that the cell was overcharged, using constant currentcycling to a level of about 4.43 volts. The flat voltage profile at 4.43volts shows the breakdown of DMAC, and the subsequent recovery of celloperation due to DMAC protection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The invention provides an understanding of the mechanisms bywhich electrolyte salt decomposes, and provides an effective method forpreventing or at least inhibiting such decomposition. Therefore, theinvention provides electrolyte stability, and particularly, thermalstability.

[0029] The exothermic reaction involving a lithiated carbon anode activematerial and the electrolyte in a cell, has not to this point beenadequately addressed. Thermal decomposition of the electrolyte salt isthought to involve, for example, LiPF₆ decomposing to LiF+PF₅. The PF₅formed is a strong oxidizing agent as well as a Lewis acid. Theoxidizing nature of the PF₅ is thought to impart significant reactivitywith lithiated carbon which itself is a very strong reducing agent. Bythe present invention, this difficulty is resolved by the addition of aspecifically selected and specifically suitable Lewis base tosignificantly reduce the reactivity by neutralizing the formed PF₅. Theaforesaid difficulty was observed in lithium ion cells having aprotic,polar solvents such as ethylene carbonate and dimethyl carbonate, incombination with a lithium ion salt, for example LiPF₆. Such electrolyteformulation, being representative of a class of solvents and salts, havedemonstrated themselves to be not stable, particularly at elevatedtemperatures such as 60 degrees centigrade. This poses a significantdisadvantage in both the processing to form the cell and the operationof the cell.

[0030] In another aspect, there is the partial decomposition of theinorganic ion salt formed in the polymer matrix. Partial decompositionoccurs due to exposure of the inorganic ion salts to the hightemperatures used, for example, in forming the polymer matrix, and/or inevaporating the volatile solvent, and/or in batteries used at elevatedtemperatures. These high temperatures cause the salt to break down intoinsoluble or less soluble salts. For example, upon decomposition oflithium hexafluorophosphate (LiPF₆), the decomposition product LiF isalso formed; and the LiF is much less soluble in the electrolyte solventand can precipitate out. Such insoluble or less soluble salts cannotfunction to transfer electrons, and hence the resulting battery isrendered less efficient.

[0031] Thus, in preparing electrolyte/separator, great care must betaken to maintain processing temperatures below the threshold level forsignificant salt decomposition. The need for careful monitoring ofprocess temperatures increases manufacturing costs and at the same timeresults in a percentage of the electrolyte/separators produced being offspecification due to unavoidable process temperature variation.Electrolyte/separator materials meeting production specificationsgenerally contain small but tolerable levels of impurities which cannevertheless affect cell performance, particularly with respect tocumulative capacity. Cumulative capacity of a battery is defined as thesummation of the capacity of the battery over each cycle (charge anddischarge) in a specified cycle life.

[0032] It has been determined that the above difficulties are overcomeby the addition of an additive Lewis base. More specifically, theadditive is a dialkylamide. It is preferred that the dialkylamide isN,N-dimethylacetamide (DMAC). The dialkylamide is effective inelectrolyte solutions comprising a solute consisting essentially of asalt of lithium, and a solvent consisting essentially of one or moreaproctic, polar solvent compounds in combination with the additive.Desirably, the polar solvent compounds are each characterized by havinga carbon connected through an oxygen to another carbon.

[0033] Preferably, the aprotic polar solvent is selected from the groupconsisting of carbonates, lactones, propionates, five member ringcompounds, and organic solvent compounds having a low alkyl group (1-4carbons) connected through an oxygen to a carbon and comprising C—O—Cbonds.

[0034] It is preferred that the aprotic, polar solvent to which thedialkylamide is added is a carbonate selected from the group consistingof propylene carbonate (PC), ethylene carbonate (EC), methyl ethylcarbonate (MEC), diethyl carbonate (DEC), dipropyl carbonate (DPC),dimethyl carbonate (DMC), butylene carbonate (BC), dibutyl carbonate(DBC), vinylene carbonate (VC), ethyl methyl carbonate (EMC), andmixtures thereof. (Table II).

[0035] The aforesaid additive was found to be effective in stabilizingthe electrolyte in cells, even at elevated temperatures. Cells whichcontained the additive were found to be stable during storage and at 0to about 4.4 volts (vs. Li/Li⁺) . Such cells included half cells formedwith graphitic active material; half cells formed with lithium metaloxide active material; and full cells of lithium metal oxide/graphite.Baseline cells having 48 centimeters squared electrodes were tested withthe additive at elevated temperature (80°) and at ambient (roomtemperature).

[0036] Advantageously, it has been found that the DMAC additive of theinvention promotes electrolyte stability during normal operation of acell, up to about 4.4 volts. In addition, the DMAC additive breaks downat potentials at or near an overcharge condition, nominally at or overabout 4.4 volts. Thus, in a condition at or near cell overchargecondition, the DMAC additive absorbs excess charge energy by degradingat or just above about 4.4 volts. This protects the active material fromelectrochemical damage by preventing the attainment of the damagedthreshold voltage for lithium metal oxide active materials such aslithium manganese oxide, lithium cobalt oxide, and lithium nickel oxide.For example, the damage threshold for lithium manganese oxide is on theorder of 4.7 volts and that of similar lithium metal oxides is in thisrange and up to about 5 volts. Therefore the DMAC additive provides twovery important advantages. It maintains stability of the electrolyte andenhances stability of other cell components in the range of the normaloperating voltage of the cell. In addition, the DMAC preventsdecomposition and degradation of cell components on an overchargecondition, since DMAC consumes excess energy generated in suchovercharge condition to prevent degradation of other cell components andto protect other cell components and particularly cathode activematerial.

[0037] Results of the testing will be described more particularly below.It should be noted that the organic acid derivative, N,N-dialkylamidewas successfully used to stabilize LiPF₆ at elevated temperature. Theinitial data provided below shows that at 80° C. the electrolyte did notchange any color in several weeks when the amide additive was included.

[0038] A battery or cell which utilizes the novel family of salts of theinvention will now be described. Note that the preferred cellarrangement described here is illustrative and the invention is notlimited thereby. Experiments based on full and half cell arrangementswere conducted as per the following description.

[0039] Polymeric electrolytic cells comprise polymeric film compositionelectrodes and separator membranes. In particular, rechargeable lithiumbattery cells comprise an intermediate separator element containing anelectrolyte solution through which lithium ions from a source electrodematerial move between cell electrodes during the charge/discharge cyclesof the cell. In such cells an ion source electrode is a lithium compoundor other material capable of intercalating lithium ions. An electrodeseparator membrane comprises a polymeric matrix made ionicallyconductive by the incorporation of an organic solution of a dissociablelithium salt which provides ionic mobility. Strong, flexible polymericelectrolytic cell separator membrane materials retain electrolytelithium salt solutions and remain functional over temperatures rangingwell below room temperature. These electrolyte membranes are used eitherin the usual manner as separator elements with mechanically assembledbattery cell components, or in composite battery cells constructed ofsuccessively coated layers of electrode and electrolyte compositions.

[0040] A typical laminated battery cell structure 10 is depicted inFIG. 1. It comprises a negative electrode side 12, a positive electrodeside 14, and an electrolyte/separator 16 therebetween. Negativeelectrode sire 12 includes current collector 18, and positive electrodeside 14 includes current collector 22. A copper collector foil 18,preferably in the form of an open mesh grid, upon which is laid anegative electrode membrane 20 comprising an intercalation material suchas carbon or graphite or low-voltage lithium insertion compound,dispersed in a polymeric binder matrix. An electrolyte separator film 16membrane of plasticized copolymer is positioned upon the electrodeelement and is covered with a positive electrode membrane 24 comprisinga composition of a finely divided lithium intercalation compound in apolymeric binder matrix. An aluminum collector foil or grid 22 completesthe assembly. Protective bagging material 40 covers the cell andprevents infiltration of air and moisture.

[0041] In another embodiment, a multicell battery configuration as perFIG. 2 is prepared with copper current collector 51, negative electrode53, electrolyte/separator 55, positive electrode 57, and aluminumcurrent collector 59. Tabs 52 and 58 of the current collector elementsform respective terminals for the battery structure.

[0042] The relative weight proportions of the components of the positiveelectrode are generally: 50-90% by weight active material; 5-30% carbonblack as the electric conductive diluent; and 3-20% binder chosen tohold all particulate materials in contact with one another withoutdegrading ionic conductivity. Stated ranges are not critical, and theamount of active material in an electrode may range from 25-85 weightpercent. The negative electrode comprises about 50-95% by weight of apreferred graphite, with the balance constituted by the binder. Atypical electrolyte separator film comprises approximately two partspolymer for every one part of a preferred fumed silica. Before It shouldbe notedremoval of the plasticizer, the separator film comprises about20-70% by weight of the composition; the balance constituted by thepolymer and fumed silica in the aforesaid relative weight proportion.The conductive solvent comprises any number of suitable solvents andsalts. Desirable solvents and salts are described in U.S. Pat. Nos.5,643,695 and 5,418,091. One example is a mixture of EC:PC:LiPF₆ in aweight ratio of about 50:44.3:5.7.

[0043] Advantageously, the additive of the invention is usable with avariety of solvents. In addition, the range of salt content may berelatively broad. Solvents are selected from such mixtures as dimethylcarbonate (DMC), diethylcarbonate (DEC), dipropylcarbonate (DPC),ethylmethylcarbanate (EMC), ethylene carbonate (EC), propylene carbonate(PC), butylene carbonate, lactones, esters, glymes, sulfoxides,sulfolanes, etc. The preferred solvents are EC/DMC, EC/DEC, EC/DPC andEC/EMC. With these combinations, there may also be used ethyl propionate(EP). Particularly preferred is EC/DMC/DMAC and EC/DMC/DMAC/EP. The saltcontent ranges from 5 to 65% by weight, preferably from 8% to 35% byweight. Physical characteristics of the DMAC are given in Table I.Physical characteristics of exemplary aprotic, polar solvents are givenin Table II. Any amount of DMAC added to the solvent is helpful.Practical amounts are up to 20% by weight of the solvent mixture,desirably up to 10%. A 0.1 to 20 weight percent is practical, 0.1 to 10%desirable, and 1 to 5% is preferred.

[0044] It should be noted that the preferred EC/DMC/DMAC provides anumber of advantages. EC is a high dielectric solvent and enhancesdissociation of the salt. DMC has low viscosity and promotes mobility ofions. DMAC enhances the thermal stability of LiPF₆ and seems tostabilize co-solvents. The same advantages apply to otherlithium-fluorine salts such as LiBF₄ and LiAsF₆.

[0045] Those skilled in the art will understand that any number ofmethods are used to form films from the casting solution usingconventional meter bar or doctor blade apparatus. It is usuallysufficient to air-dry the films at moderate temperature to yieldself-supporting films of copolymer composition. Lamination of assembledcell structures is accomplished by conventional means by pressingbetween metal plates at a temperature of about 120-160° C. Subsequent tolamination, the battery cell material may be stored either with theretained plasticizer or as a dry sheet after extraction of theplasticizer with a selective low-boiling point solvent. The plasticizerextraction solvent is not critical, and methanol or ether are oftenused.

[0046] Separator membrane element 16 is generally polymeric and preparedfrom a composition comprising a copolymer. A preferred composition isthe 75 to 92% vinylidene fluoride with 8 to 25% hexafluoropropylenecopolymer (available commercially from Atochem North America as KynarFLEX) and an organic solvent plasticizer. Such a copolymer compositionis also preferred for the preparation of the electrode membraneelements, since subsequent laminate interface compatibility is ensured.The plasticizing solvent may be one of the various organic compoundscommonly used as solvents for electrolyte salts, e.g., propylenecarbonate or ethylene carbonate, as well as mixtures of these compounds.Higher-boiling plasticizer compounds such as dibutyl phthalate, dimethylphthalate, diethyl phthalate, and tris butoxyethyl phosphate areparticularly suitable. Inorganic filler adjuncts, such as fumed aluminaor silanized fumed silica, may be used to enhance the physical strengthand melt viscosity of a separator membrane and, in some compositions, toincrease the subsequent level of electrolyte solution absorption.

[0047] In the construction of a lithium-ion battery, a current collectorlayer of aluminum foil or grid is overlaid with a positive electrodefilm, or membrane, separately prepared as a coated layer of a dispersionof intercalation electrode composition. This is typically anintercalation compound such as LiMn₂O₄ (LMO), LiCoO₂, or LiNiO₂, powderin a copolymer matrix solution, which is dried to form the positiveelectrode. An electrolyte/separator membrane is formed as a driedcoating of a composition comprising a solution containing VdF:HFPcopolymer and a plasticizer solvent is then overlaid on the positiveelectrode film. A negative electrode membrane formed as a dried coatingof a powdered carbon or other negative electrode material dispersion ina VdF:HFP copolymer matrix solution is similarly overlaid on theseparator membrane layer. A copper current collector foil or grid islaid upon the negative electrode layer to complete the cell assembly.Therefore, the VdF:HFP copolymer composition is used as a binder in allof the major cell components, positive electrode film, negativeelectrode film, and electrolyte/separator membrane. The assembledcomponents are then heated under pressure to achieve heat-fusion bondingbetween the plasticized copolymer matrix electrode and electrolytecomponents, and to the collector grids, to thereby form an effectivelaminate of cell elements. This produces an essentially unitary andflexible battery cell structure.

[0048] Examples of forming cells containing metallic lithium anode,intercalation electrodes, solid electrolytes and liquid electrolytes canbe found in U.S. Pat. Nos. 4,668,595; 4,830,939; 4,935,317; 4,990,413;4,792,504; 5,037,712; 5,262,253; 5,300,373; 5,435,054; 5,463,179;5,399,447; 5,482,795 and 5,411,820; each of which is incorporated hereinby reference in its entirety. Note that the older generation of cellscontained organic polymeric and inorganic electrolyte matrix materials,with the polymeric being most preferred. The polyethylene oxide of U.S.Pat. No. 5,411,820 is an example. More modern examples are the VDF:HFPpolymeric matrix. Examples of casting, lamination and formation of cellsusing VdF:HFP are as described in U.S. Pat. Nos. 5,418,091; 5,460,904;5,456,000; and 5,540,741; assigned to Bell Communications Research, eachof which is incorporated herein by reference in its entirety.

[0049] As described earlier, the electrochemical cell which utilizes thenovel solvent of the invention may be prepared in a variety of ways. Inone embodiment, the negative electrode may be metallic lithium. In moredesirable embodiments, the negative electrode is an intercalation activematerial, such as, metal oxides and graphite. When a metal oxide activematerial is used, the components of the electrode are the metal oxide,electrically conductive carbon, and binder, in proportions similar tothat described above for the positive electrode. In a preferredembodiment, the negative electrode active material is graphiteparticles. For test purposes, test cells were fabricated using lithiummetal electrodes. When forming cells for use as batteries, it ispreferred to use an intercalation metal oxide positive electrode and agraphitic carbon negative electrode. Various methods for fabricatingelectrochemical cells and batteries and for forming electrode componentsare described herein. The invention is not, however, limited by anyparticular fabrication method as the novelty lies in the uniqueelectrolyte.

EXAMPLE I

[0050] A graphite electrode was fabricated by solvent casting a slurryof BG-35 graphite, binder, plasticizer, and casting solvent. Thegraphite, BG-35, was supplied by Superior Graphite, Chicago, Ill. The BGseries is a high purity graphite derived from a flaked natural graphitepurified by heat treatment process. The physical features are given inTable III. The binder was a copolymer of polyvinylidene difluoride(PVDF) and hexafluoropropylene (HFP) in a wt. ratio of PVDF to HFP of88:12. This binder is sold under the designation of Kynar Flex 2801®,showing it's a registered trademark. Kynar Flex is available fromAtochem Corporation. An electronic grade solvent was used. The slurrywas cast onto glass and a free standing electrode was formed as thecasting solvent evaporated. The slurry composition was as follows:Component Wet Weight % Dry Weight % Graphite 25.0 60.0 Binder 6.8 16.4Plasticizer 8.9 21.4 Carbon 0.9 2.2 Solvent 58.4 — Total 100.0 100.0

[0051] The counter-electrode was lithium metal. A glass fiber separatorwas used between the electrodes to prevent them from electricallyshorting. An electrochemical cell of the first electrode, separator, andcounter-electrode was formed.

[0052] The electrolyte used to form the completed final cell or batterycomprised a solution of ethylene carbonate (EC), dimethyl carbonate(DMC), and N,N-dimethylacetamide (DMAC). Two different amounts of theadditive were tested. One was 1% DMAC and 99% of the EC/DMC. The otherwas 10% DMAC and 90% of the EC/DMC. In both cases the EC/DMC weightratio was 2:1. The electrolyte solution contained 1 molar LiPF₆ salt.The two electrodes were maintained in separated condition using a glassfiber layer. The electrolyte solution interpenetrated the void spaces ofthe glass fiber layer. The results of constant current cycling are shownin FIG. 3. FIG. 3 shows a voltage/capacity plot of BG-35 graphite cycledwith a lithium metal electrode using constant current cycling at ±0.2milliamps per square centimeter, between 0.01 and 2.0 volts versusLi/Li⁺, using 19 milligrams of the BG-35 active material. Theelectrolyte is 1 molar LiPF₆ in a solution of 90% by weight of 2:1EC/DMC and 10% by weight DMAC. In the first half cycle, lithium isremoved from the metallic electrode and intercalated into the graphiteelectrode. When essentially full intercalation at the graphite electrodeis complete, corresponding to about Li₁C₆, the voltage has dropped toapproximately 0.01 volts, representing about 370 milliamp hours pergram, corresponding to about 7.1 milliamp hours based on 19 milligramsof active material. In the second half cycle, the lithium isdeintercalated from the graphite and returned to the metallic electrodeuntil the average voltage is approximately 2 volts versus Li/Li⁺. Thedeintercalation corresponds to approximately 318 milliamp hours pergram, representing approximately 6.1 milliamp hours based on 19milligrams of active material. This completes an initial cycle. Thepercentage difference between the 370 milliamp hours per gram capacity“in”, and the 318 milliamp hours per gram capacity “out”, divided by theinitial capacity “in”, corresponds to a surprisingly low 14 percentloss. In the rest of FIG. 1, the cycling is repeated, maintaining highcapacity.

EXAMPLE II

[0053] An electrode cathode was also fabricated by solvent casting aslurry of lithium manganese oxide, conductive carbon, binder,plasticizer, and solvent. The lithium manganese oxide used was LiMn₂O₄supplied by Kerr-McGee (Soda Springs, Id.); the conductive carbon usedwas Super P (MMM carbon), Kynar Flex 2801® was used as the binder alongwith a plasticizer, and electronic grade acetone was used as thesolvent. The slurry was cast onto aluminum foil coated with polyacrylicacid/conductive carbon mixture. The slurry was cast onto glass and afree standing electrode was formed as the solvent was evaporated. Thecathode slurry is composition was as follows: Component Wet Weight % DryWeight % LiMn₂O₄ 28.9 65.0 Graphite 2.5 5.5 Binder 4.5 10.0 Plasticizer8.7 19.5 Solvent 55.4 — Total 100.0 100.0

[0054] The cell was prepared as noted above. The electrochemical cellwas prepared as noted above with respect to Example I. The electrolytewas prepared having the same composition as the electrolyte of ExampleI.

[0055]FIG. 4 contains the results of constant current cycling and is agraph of cell voltage versus capacity. FIG. 4 shows a voltage/capacityplot of lithium manganese oxide (nominally LiMn₂O₄, Li_(1+x)Mn₂O₄,−0.2≦x≦0.2; usually x is 0.6 normal starting stoichiometry for LMO)cycled with a lithium metal electrode using constant current cycling at±0.2 milliamps per square centimeter, between about 3 and 4.3 voltsversus Li/Li⁺, using 30 milligrams of the LMO active material. Theelectrolyte is 1 molar LiPF₆ in a solution of 90% by weight of 2:1EC/DMC and 10% by weight DMAC.

[0056] In an as-assembled, initial condition, the positive electrodeactive material is nominal LiMn₂O₄. The lithium is deintercalated fromLMO during charging of the cell. When fully charged, optimally about 0.8unit of lithium has been removed per formula unit of the originalLiMn₂O₄. In this fully charged condition, the electrochemical potentialversus lithium of the LMO, is about 4.3 volts. The deintercalation oflithium from LMO results in approximately 135 milliamp hours per gramcorresponding to 4.1 milliamp hours. Next, the cell is dischargedwhereupon a quantity of lithium is reintercalated into the LMO. Thereintercalation corresponds to approximately 122 milliamp hours per gramor 3.7 milliamp hours, and the bottom of the curve corresponds toapproximately 3 volts. The cell is then subsequently recharged whereupona quantity of lithium ions is again deintercalated, returning to theregion of approximately 4 volts. Charging and discharging continuedsuccessfully over a number of additional cycles. As can be seen fromFIG. 4, the first cycle loss corresponded to only 10 percent, which isvery good.

EXAMPLE III

[0057] In this example, a rocking chair battery was prepared comprisinga graphite anode, an intercalation compound cathode, and a novelelectrolyte comprising the DMAC additive of the invention. The negativeelectrode comprising BG-35 was prepared as described in Example I. Thelithium manganese oxide positive electrode was prepared in accordancewith the description given in Example II. The active mass of thenegative electrode was 570 milligrams and the active mass of thepositive electrode was 1710 milligrams. A first solvent solution of 1molar LiPF₆ in EC/DMC (2:1 by weight) was prepared. Then, 99 percent byweight of this first solvent solution was mixed with 1 percent by weightof DMAC, to form a solvent mixture for the test cell. (EC/DMC/DMAC at66:33:1 weight ratio.) The two electrode layers were arranged with anelectrolyte layer in between, and the layers were laminated togetherusing heat and pressure as per the Bell Comm. Res. patents incorporatedherein by reference earlier.

[0058]FIG. 5 is a two-part graph. FIG. 5A shows the excellentrechargeability and FIG. 5B shows the excellent cyclability and capacityof the cell prepared in accordance with Example III. The capacity wasdetermined at constant current cycling for cycles 1 to 107 consistentwith the test parameters described above. FIG. 5 shows long cycle lifedemonstrated by the relatively slow capacity fade with cycle numbers.The recharge ratio data shows the absence of any appreciable sidereactions and decompositions over the extended life cycling. This can bemore particularly seen from FIG. 5A. The recharge ratio maintains itsvalue exceptionally close to 1. The cell maintains over 92 percent ofits capacity over extended cycling to 100 cycles. The combination ofslow, minimal capacity fade along with excellent recharge ratiodemonstrates the absence of any appreciable side reactions. The cell ofFIG. 5 contained 1 M LiPF₆ EC/DMC (2:1 wt.) with 1% DMAC addition. Itcycled well with low capacity fade. It indicated a good compatibility ofthe DMAC in the system which stabilized the electrolyte. As per ExamplesI and II, the use of DMAC as an additive stabilizes the solvent mixtureagainst breakdown.

COMPARATIVE EXAMPLE

[0059] For comparison purposes, an additional cell was prepared inaccordance with the methods of Examples I, II and III, except that thesolvent did not contain any DMAC. The solvent was 1 molar LiPF₆ in 2:1EC/DMC. This electrolyte was also used in a cell having a lithium metaloxide positive electrode and a BG-35 negative counter-electrode. Theactive mass of the positive electrode was 1524 milligrams and thenegative electrode was 508 milligrams. This comparative cell withoutDMAC additive was also stored at 80° C. for 27 days.

[0060]FIG. 6 contains the results of cycling the comparative cell(without DMAC) and repeats the performance of the cell of Example III(1% DMAC) for direct comparison. The dashed lines of FIGS. 6A and 6Bshow the poor performance of the comparative cell. The cells of FIG. 6were activated using electrolytes (with or without DMAC addition) thathad been stored for 27 days at 80° C. The cell (solid line) with DMACaddition during storage showed 16% first cycle loss (table IV) andmaintained 92% of initial capacity at 100 cycles. The cell without DMAC(dashed --), however, gave 39% first cycle loss with 80% initialcapacity at 100 cycles. This is evidence that electrolyte breakdown isnot occurring. Further evidence of lack of electrolyte breakdown is thefact that the cell does not expand in volume and puff up. This showsabsence of gas formation caused by electrolyte breakdown. Absence ofsuch gassing, absence of electrolyte breakdown, and absence ofirreversible charge consumption demonstrates the unique and unexpectedadvantage of the electrolyte solvent of the invention.

[0061] Further data showing the beneficial effects of adding DMAC toelectolyte solution are given in Tables IV and V. Table IV clearly showsthat with DMAC, the stored electrolyte shows little solution colorchange. In contrast, the same electrolyte solution without DMAC showssignificant color change demonstrating electrolyte degradation.

[0062] Table V shows results of DSC reactivity of anode films for fullyintercalated carbon electrode (i.e., the anode potential (20 mV vs.Li/Li⁺). The thermal reactivity of the anode toward the electrolyte issignificantly improved with inclusion of DMAC in the electrolytesolution. The 55-86 (−J/g) exotherm for the BG-35/EC-DMC-DMACcombination is a fraction of the 550 (−J/g) for the EC/DMC combination.

EXAMPLE IV

[0063] A constant current cycling test was conducted and an overchargecondition was induced to demonstrate the ability of the DMAC to protectcell components during a non-design, overcharge event. The conditions oftesting are the same as that described above for FIG. 4, except that thecell was overcharged, using constant current cycling to a level of about4.43 volts. The cell contained lithium manganese oxide electrode cycledwith a lithium metal counter-electrode using constant current cycling.The electrolyte was 1 molar LiPF₆ in a solution of ethylene carbonate(EC) and dimethyl carbonate (DMC), 2:1 wt of EC:DMC; and, 10% by weightdimethylacetamide (DMAC) was added to the aforesaid solution.

[0064]FIG. 7 shows the results of the overcharge test. FIG. 7 clearlydemonstrates the protection of lithium manganese oxide by DMAC additionto the electrolyte. The flat voltage profile at 4.43 volts shows thebreakdown of DMAC and the cathode capacity upon discharge or chargeremains unchanged during subsequent cycles as shown in FIG. 7.

[0065] More specifically, FIG. 7 shows that once the voltage is broughtup to about 4.3 to 4.4 volts or greater, a flat plateau occurs. Thisflat plateau indicates the breakdown of the DMAC. Subsequently, ondischarge, 2.9 milliamp hours results showing recovery of the cell. Thisovercharge condition was induced a second time whereupon, additionalDMAC absorbed the energy of excess charge and broke down againprotecting cell components. Thereupon, a discharge value of 3.0 milliamphours was obtained. This demonstrated that even though the cell wascycled to an overcharge condition causing breakdown of some of the DMAC,the cell was able to recover and cycle to subsequent overcharge anddischarge conditions. This will continue to occur as long as DMAC isavailable to absorb the excess charge potential. Therefore, these shortoccurrences of overcharge caused breakdown of DMAC, instead of breakingdown precious cell components such as cathode active material.Therefore, although active material such as lithium manganese oxide canbe electrochemically damaged if the potential is above 5 volts, theaddition of DMAC into the electrolyte protects the active material frombeing overcharged since the DMAC is first broken down at a lowerpotential (4.43 volts) absorbing the excess energy before the cellreaches the potential of the active material breakdown. TABLE IN,N-dimethylacetamide (DMAC) Physical: Colorless liquid SpecificGravity: 0.9366 at 25° C. Melting Point: −57° C. Boiling Point at 760mm: 163-165° C.

[0066] TABLE II Characteristics of Organic Solvents PC VC EC DMC BoilingTemperature (C) 240 162 248 91.0 Melting Temperature (C) −49 22 39-404.6 Density (g/cm³) 1.198 1.35 1.322 1.071 Solution Conductivity (S/cm)2.1 × 109 — <10⁻⁷ <10⁻⁷ Viscosity (cp) at 25° C. 2.5 — 1.86 (at 40° C.)0.59 Dielectric Constant at 20° C. 64.4 — 89.6 (at 40° C.) 3.12Molecular Weight 102.0 86.047 88.1 90.09 H₂O Content <10 ppm — <10 ppm<10 ppm Electrolytic Conductivity 5.28 — 6.97 11.00 (mS/cm) 20° C. 1MLiAsF₆ (1.9 mol) Boiling Temperature (C) 126 230 107 167-168 MeltingTemperature (C) −43 — −55 — Density (g/cm³) 0.98 1.139 1.007 0.944Solution Conductivity (S/cm) <10⁻⁷ <10⁻⁷ 6 × 10⁻⁹ <10⁻⁷ Viscosity (cp)at 25° C. 0.75 2.52 0.65 — Dielectric Constant at 20° C. 2.82 — — —Molecular weight 118.13 116.12 104.10 146.19 H₂O Content <10 ppm <10 ppm<10 ppm <10 ppm Electrolytic Conductivity 5.00 <3.7 — — (mS/cm) 20° C.1M LiAsF₆ (1.5 mol)

[0067] TABLE III Carbon Material BG-35 Surface Area (m²/g) (BET) 7Coherence Length L_(c) (nm) >1000 Density (g/cm³)² 0.195 Particle Size¹<36 Median Size d₅₀ (μm) 17 Interlayer Distance c/2 (nm) N/A

[0068] TABLE IV Storage Test Results of Electrolyte Solution With andWithout DMAC Addition First cycle loss Electrolyte solution BG35/LMOcells with the 27 day Appearance after 10 Appearance after 27 stored (%)days at 80° C. days at 80° C. electrolyte 1.0 M LiPF₆ clear, colorlesslight brown 16 EC/DMC (2:1 wt.) with 1% DMAC 1.0 M LiPF₆ light browndark brown 39 EC/DMC (2:1 wt.)

[0069] TABLE V Effects of DMAC to the Anode Reactivity TowardElectrolyte Anode Exothermic Exotherm (−J/g) Electrolyte solutionformulation temperature (° C.) 1.0 M LiPF₆ 60% BG 35 145-146 55-86EC/DMC (2:1 wt.) with 1% DMAC 1.0 M LiPF₆ 60% BG 35 132-148 >500 EC/DMC(2:1 wt.)

[0070] In summary, the invention solves the problems associated withconventional electrolytes. Solvents containing DMC have always been aproblem since DMC readily boils off. EC readily solidifies, and it isnecessary for the cell to achieve a temperature of 40° C. to melt the ECand prevent it from solidifying. In addition, mixtures of DMC/EC havebeen found to result in decomposition evidenced by solution color changeand/or by formation of gas. In contrast, solvents of the invention whichinclude DMAC additive provide highly desirable wide temperatureoperating range while avoiding decomposition of cell components. Whilenot wishing to be held to any particular theory, it is thought that thesolvent of the invention avoids problems associated with conventionalsolvents containing C—O—C bonds. The solvent of the present inventionappears to stabilize the lithium metal salt and/or the C—O—C bond. It isalso possible that the DMAC neutralizes HF formed from reaction of LiPF₆with water. It is believed that HF catalyzes decomposition of LiPF₆ saltat elevated temperature. The nitrogen of DMAC acts as a proton acceptor,therefore DMAC is a Lewis base. The DMAC additive thus lessens theextent of decomposition of the lithium salt. Therefore, the ionicspecies of the salt are preserved. In the exemplary LiPF₆, the lithiumion Li⁺ and corresponding anion (counter ion) PF⁻ ₆ are each maintained.Formation of the exemplary decomposition product, LiF, is prevented orat least reduced. Therefore, the solvents of the invention are animprovement over conventional solvents. It is thought that the solventsof the invention also help overcome problems associated with reactiveactive materials and avoids the consequences of catalytic reaction whichcatalyzes decomposition of electrolyte solvent.

[0071] The DMAC additive provides two very important advantages. Itmaintains stability of the electrolyte cell components in the range ofthe normal operating voltage of the cell. In addition, the DMAC preventsdecomposition and degradation of cell components on an overchargecondition, since DMAC consumes excess energy generated in suchovercharge condition to prevent degradation of other cell components andto protect other cell components and particularly cathode activematerial. Therefore, it is thought that the stable electrolyte solventof the invention provides a significant advantage since it avoidsdecomposition caused by a variety of mechanisms.

[0072] While this invention has been described in terms of certainembodiments thereof, it is not intended that it be limited to the abovedescription, but rather only to the extent set forth in the followingclaims.

[0073] The embodiments of the invention in which an exclusive propertyor privilege is claimed are defined in the following claims.

1. An electrochemical cell having an electrolyte which comprises asolute, a solvent, and an additive; said solute consisting essentiallyof a salt of lithium; said solvent consisting essentially of one or moreaprotic, polar solvents, and said additive being a dialkylamide.
 2. Thecell according to claim 1 wherein said dialkylamide isN,N-dimethylacetamide (DMAC).
 3. The cell according to claim 1 whereinsaid aproctic, polar solvent is selected from the group consisting ofcarbonates; lactones; propionates; five member heterocyclic ringcompounds; and organic solvent compounds having a low alkyl (1-4 carbon)group connected through an oxygen to a carbon, and comprising C—O—Cbonds.
 4. The cell according to claim 1 wherein the carbonate isselected from the group consisting of propylene carbonate (PC), ethylenecarbonate (EC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC),dipropyl carbonate (DPC), dimethyl carbonate (DMC), butylene carbonate(BC), dibutyl carbonate (DBC), and vinylene carbonate (VC).
 5. The cellaccording to claim 1 wherein said one or more aprotic, polar solventcompounds has a carbon connected through an oxygen to another carbon,and said additive being an N,N-dialkylamide.
 6. The cell according toclaim 1 wherein said solvent consists of ethylene carbonate (EC),dimethyl carbonate (DMC) and dimethylacetamide (DMAC).
 7. The cellaccording to claim 1 wherein said solvent consists of ethylene carbonate(EC), dimethyl carbonate (DMC) dimethylacetamide (DMAC), and ethylpropionate (EP).
 8. The cell according to claim 2 wherein said DMAC ispresent in an amount by weight of up to 20% of said solvent.
 9. The cellaccording to claim 8 wherein said DMAC is present in an amount by weightof 0.1% to 5% of said solvent.
 10. A method for reducing decompositionof an electrolyte solution and for reducing the formation of gaseousconstituents in an electrochemical cell, said method comprisingincluding in said cell a dialkylamide additive, whereby said cell havingsaid additive is characterized by a lesser rate of gas formation duringcycling of said cell as compared to a similar cell without saidadditive.
 11. A method for reducing decomposition of a lithium salt inan electrochemical cell, said method comprising including in said cell adialkylamide additive which neutralizes acid attack of said salt.
 12. Amethod for preventing breakdown of a lithium metal oxide cathode activematerial in an electrochemical cell by overcharge to an electrochemicalbreakdown voltage, said method comprising, including in said cell DMAC(dimethylacetamide) which absorbs excess charge energy at a voltage lessthan the breakdown voltage of said cathode active material.
 13. Themethod of claim 12 wherein the active material is lithium manganeseoxide having a breakdown voltage of about 5 volts and said DMACcharacterized by absorbing excess energy at a breakdown voltage lessthan that of said lithium manganese oxide.